In a typical surface mount electronic circuit board manufacturing facility, a number of independent machines, each of which performs a different operation, are used in an assembly line to manufacture circuit boards. One typical example of a circuit board assembly line 10 is shown in block format in FIG. 1. The assembly line includes a circuit board loader 12, a stencil printer 14, a pick and place machine 16 and a reflow oven 18. The machines in the assembly line are interconnected using a number of conveyor segments 20.
In the assembly line 10, the circuit board loader 12 provides a bare circuit board to the stencil printer wherein solder paste is printed onto pads of the circuit board in a predetermined pattern defined by a stencil loaded into the stencil printer. The stencil printer then passes the circuit board to the pick and place machine 16, wherein electrical components are placed onto the solder paste on the circuit board. Next, the circuit board is passed to the reflow oven, wherein the circuit board is subjected to a sufficiently high temperature to cause the solder paste to flow to secure the components to the board and provide proper electrical connections between the components and the circuit board.
One example of a stencil printer 14 is shown in FIG. 2. Stencil printers, like that shown in FIG. 2, are available from MPM Corporation, Franklin MA, the assignee of the present invention, and include model numbers Ultraprint 2000, Ultraprint 2500 and Ultraprint 3000. The stencil printer 14 includes a frame 22 that supports components of the stencil printer including a controller 24 with a user interface 26, a stencil 28, and a dispensing head 30 having a dispensing slot 32 from which solder paste is dispensed. When a circuit board 36 is loaded into the stencil printer and properly aligned in the printer, a z-axis work table lifts the circuit board to place the circuit board against, or in close proximity with, the stencil of the printer so that printing can occur. The dispensing head 30 is then placed over the stencil and moved across the stencil while solder paste is dispensed through the slot 32, causing the solder paste to be printed onto the circuit board. The operation of the dispensing head 30 is further described in co-pending U.S. patent application No. 08/966,057 to Freeman et. al, which is assigned to the assignee of the present invention, and incorporated by reference herein. As known to those skilled in the art, some prior art stencil printers utilize one or more squeegee blades with a paste dispensing mechanism in place of the dispensing head described above.
The stencil printer 14 includes a conveyor 34 for loading/unloading and for positioning circuit boards in a print area above the z-axis work table of the screen printer. The conveyor 34 of the stencil printer is positioned to receive circuit boards from and deliver circuit boards to conveyor sections 20 of an assembly line, as shown in FIG. 1, to move circuit boards between the machines in the assembly line. Typical inline circuit board manufacturing machines, such as those shown in FIG. 1, communicate with each other using standard protocols defined by the Surface Mount Equipment Manufacturer's Association (SMEMA) to transfer substrates such as circuit boards between the machines. For example, using SMEMA protocols, the stencil printer 14 signals to the pick and place machine that it has a circuit board ready to transfer to the pick and place machine, and when the pick and place machine signals to the stencil printer that it is ready to receive the circuit board, then the circuit board is transferred over conveyor section 20.
To reduce the time and conveyor travel distance required to load and unload circuit boards, in some prior art stencil printers, conveyor 34 in the stencil printer is comprised of three independently controllable conveyor segments 34a, 34b and 34c. The three segments define three circuit board positions identified as a pre-print position 38, a print position 40 and a post-print position 42. Circuit boards are initially loaded into the pre-print position 38, move to the print position 40 within the print area 44 of the screen printer, and after printing, move to the post-print position 42 to wait to be unloaded from the stencil printer. The use of independently controllable conveyor segments allows circuit boards to be loaded and unloaded in a more flexible manner to meet demands of the other machines of an assembly line without waiting for a print cycle to be completed, thereby lowering the idle time of the machines, and allowing the assembly line to operate in an asynchronous manner.
Although the use of three conveyor segments for the conveyor in a stencil printer has advantages over the use of a single conveyor segment, this approach is relatively expensive, is complex and typically requires a significant amount of space for the mechanical and electrical components used to implement the three conveyor segment approach.